The electrochemical etch-stop technique is an attractive method for fabricating microsensors and microactuators since it has the potential for allowing one to reproducibly fabricate moderately-doped n-type silicon microstructures with good thickness control. A number of investigators have reported on the fabrication of silicon microstructures using electrochemical etch-stopping in various anisotropic etchants (KOH, ethylenediamine pyrocatechol, and hydrazine). The term "anisotropic etchant" means, in these etchant the (111) silicon plane etches very slowly compared to both (100) and (110) plane.
The working principle of the conventional junction electrochemical etch-stop technique is illustrated in FIG. 1A which shows the three-electrode etching configuration for fabricating silicon membranes. The three electrodes are: a (100) Si wafer working electrode 10 with a p-n junction 12 formed at the interface of a p-layer 14 and an n-layer 16, a platinum counterelectrode 18, and a reference electrode 20. A mask 22 is selectively formed on the Si wafer. The electrodes are submerged in a wet etchant 24 within a container 26. A constant positive bias voltage is applied to the silicon wafer with respect to the reference electrode. Due to the presence of the reverse-bias junction, no current can flow to the p-layer and it will not be affected by the bias voltage. Therefore, the p-layer will be anisotropically etched. The term "anisotropically etched" means etching only proceeds in the direction perpendicular to the surface of the silicon wafer, i.e., in the (100) direction, but does not proceed in one direction parallel to the surface, (111) direction (as illustrated in FIG. 1B). As soon as the p-layer is etched off and the n-layer is exposed to the etchant (FIG. 1B), the n-layer will be passivated by the constant positive bias voltage and etching will stop. A thin moderately-doped n-type membrane is thus formed.
A major challenge in using electrochemical etch-stop is to maintain the positive bias voltage applied to the silicon wafer sufficiently anodic to passivate the entire n-layer, so that the yield of this process can be maximized. In order to precisely control the amplitude of the positive bias voltage, a reference electrode is needed (see FIGS. 1A and 1B).
A reference electrode consists of a metal and a compound of the same metal, with which a very reproducible equilibrium potential can be established. When a reference electrode is used in the electrochemical etch system (FIG. 1A), the potential difference between the silicon working electrode and the reference electrode is continuously measured and feedbacked to the control system (a potentiostat). Therefore, the positive bias voltage applied to the silicon wafer can be precisely controlled.
Commercially available reference electrodes, such as silver-silver chloride (Ag/AgCl) 28 and mercury/mercury oxide (Hg/HgO), are contained inside glass or plastic tubes 30. The tubes are filled with special electrolytes 32 such as KCl (see FIG. 2). A porous glass fret 34 was then used to connect the internal electrolyte to the etchant 36. The reasons why separate tubes are needed for the reference electrodes are described as follows.
For the Ag/AgCl reference electrode 28, potassium chloride electrolyte 32 is necessary to establish a reproducible equilibrium potential between silver and silver chloride, and the pair is not stable in the caustic etchants used for the electrochemical etch-stop process. Therefore, the Ag/AgCl reference electrode has to be contained inside a separate tube 30 filled up with potassium chloride electrolyte. For the mercury/mercury oxide reference electrode, the pair is able to establish a stable equilibrium potential in caustic etchants. However, mercury is considered as a hazardous material, and it is desirable to contain such hazardous material in a separate tube.
If a Ag/AgCl or Hg/HgO reference electrode is used for the electrochemical etch-stop process, the caustic etchant used to etch the silicon will gradually degrade the glass fret. Eventually the glass fret will disintegrate, and the reference electrode needs to be replaced. Continuously monitoring the condition of a reference electrode could be troublesome for mass producing silicon microstructures using the electrochemical etch-stop process. Therefore, a relatively maintenance free reference electrode, i.e., without using a glass tube and fret, is desirable.
The present invention overcomes the problems in the prior art.